How did Shannon Reynolds make it through several layers of security unquestioned and end up on the tarmac at Hartsfield-Jackson Atlanta International Airport?

That's what Delta Air Lines, the Transportation Security Administration and the Atlanta Police Department are trying to figure out.

Around 5 a.m. on February 23, Reynolds was simply trying to park at an off-site parking lot in time to catch a flight on Spirit Airlines, she told CNN.

She didn't notice she was pulling into a Delta employee parking lot, where she said she was waved through two security checkpoints and allowed to park. Wearing a sundress and carrying her suitcase, she got on a Delta employee shuttle without being asked for identification. None of the employees on board was wearing a uniform, she said, so she said she didn't realize it was for employees only.

"When I got to the airport, I looked around and I realized this is not where I was supposed to be. It was the tarmac with a big airplane sitting there," Reynolds said.

Reynolds alerted the shuttle driver, but she was still allowed to get out of the bus onto the tarmac. A ramp agent used his badge to take her up the stairs into the airport passenger area.

"Here I was sitting in the middle of the C Terminal without having to go through security or do anything," Reynolds said. The airport employee who let her into the secure area told her to go downstairs and go through security as she normally would, Reynolds said. Inside the terminal someone finally stopped her and asked for her identification.

That started the investigation into how she had made it so far without being checked. (In a police report on the incident, the ramp agent she followed into the concourse said he assumed she was an employee because of all the checkpoints she had to pass.)

"Airports are responsible for implementing security programs that restrict access to secure areas," said TSA spokesman David Castelveter in a statement. "TSA is conducting an investigation to determine how an individual without proper credentials accessed the secure area of Hartsfield-Jackson Atlanta International Airport via an airline employee parking lot."

Delta is re-emphasizing the checking of employee identification at Delta employee parking lots, according to the airline.

"We have taken aggressive action with our security vendors at the parking lot to emphasize that all security procedures, especially verification of credentials, are strictly adhered to," said Delta spokesman Morgan Durrant in a statement.

The police report states that Reynolds "was transported back to her vehicle and allowed to move her car" from the parking lot, noting that the delay caused her to miss her 6:30 a.m. flight to Fort Lauderdale, Florida, then on to Costa Rica.

Reynolds told CNN she was able to catch a later flight to Costa Rica, from where she recently returned.